Frequency responsive reed decoder with low sensitivity to physical shock



Oct. 28. 1969 GUNN 3,475,728

FREQUENCY RESPONSIVE REED DECODER WITH Low SENSITIVITY TO PHYSICAL SHOCK Filed April 24, 1967 2 Sheets-Sheet 1 IO |Z |4 |6 |8 20 22 24 RIF CONVERTER LIMITER DISC. AUDIO AMP AMF? T AMF? ATTYS.

I34 F '30 5+ i I38 Oct. 28. 1969 D. L. GUNN 3,475,728

' FREQUENCY RESPONSIVE REED DECODER WITH LOW SENSITIVITY TO PHYSICAL SHOCK Filed April 24, 1967 2 Sheets-Sheet 2 L)% AS [H8 :IOZ r O ?=OV I36; 140 132 5+ o. J

United States Patent FREQUENCY RESPONSIVE REED DECODER WITH LOW SENSITIVITY TO PHYSICAL SHOCK David L. Gunn, Lombard, Ill., assignor to Motorola, Inc., Franklin Park, Ill., a corporation of Illinois Filed Apr. 24, 1967, Ser. No. 633,247 Int. Cl. H04m 11/02; H04q 7/02 US. Cl. 340-164 Claims ABSTRACT OF THE DISCLOSURE The decoder includes a resonant reed device having a driver winding for coupling an applied signal at the resonant frequency of the device to its pick-up winding. Opposite phases derived from opposite ends of the secondary winding are combined with the applied signal to provide sum and difference signals. A detector generates a pair of DC voltages respectively related to the amplitude of the sum and difference signals, and when the difference between the voltages reaches a predetermined value, a switch is actuated to enable an alerting device.

Background of the invention In selective signalling equipment, a decoder circuit is provided to render an oscillator operative to provide an alerting tone in response to reception of tone signals of the proper frequency. It has become quite common to use frequency selective vibratory reed devices in such decoder circuits because such devices provide a very sharp response to select signals of one frequency from signals of closely adjacent frequencies. Miniature reed devices have been used in portable units, such as paging equipment, which when dropped or otherwise subjected to shock, can cause movement of the reeds and produce a false alerting tone.

Summary It is an object of this invention to provide a decoder circuit for a reed selective system with improved insensitivity to mechanical shock.

Another object is to provide a decoder circuit for portable selective signalling equipment having means forming an integral part of the circuit for reducing the possibility of an alerting tone being generated when the equipment is physically shocked.

In practicing a preferred form of the invention, there is provided a decoder circuit which may be used in a radio receiver to provide an alerting tone upon the reception of a tone signal within a limited range of frequencies centered about a selected frequency. The decoder circuit includes a reed selective amplifier coupled to the output of the modulation detector of the receiver to apply the tone signal to the driver winding of a vibratory reed device. If the frequency of the tone signal is near the resonant frequency of the reed device, or if the device is shock excited, oppositely phased signals at such frequency respectively appear at opposite ends of the pickup winding of the device. A circuit coupled to the modulation detector bypasses the reed device to combine the tone signal with the oppositely phased signals to provide sum and difference signals. A tone frequency detector has a pair of channels respectively coupled to the ends of the pickup winding to provide direct current voltages respectively related to the magnitude of the sum and difference signals. The direct current voltages are differentially combined in a switch which generate a control signal of one value when the frequency of the tone signal is within the limited range of frequencies and another value when the frequency of the tone signal is outside such range or when the reed device is shock excited. Such control signal may be used to enable a tone producing oscillator to in turn provide an alerting tone.

Brief description of the drawings FIG. 1 is a schematic diagram of the decoder circuit of the invention coupled to a receiver, with stages of the receiver being shown in block diagram;

FIG. 2 illustrates waveforms produced in the circuit when the tone signal is at the resonant frequency of the vibratory reed device;

FIG. 3 illustrates the frequency response of the vibratory reed device of FIG. 1;

FIG. 4 illustrates circuit waveforms when the tone signal frequency is removed from the resonant frequency of the reed device;

FIG. 5 illustrates circuit waveforms when the tone signal frequency differs substantially from the resonant frequency of the reed device;

FIG. 6 illustrates circuit waveforms when the reed device is shock excited; and t FIG. 7 illustrates a second embodiment of the inven- 1011.

Detailed description of the preferred embodiments Referring now to the drawing, the decoder circuit of the invention illustrated in FIG. 1 may be used in a frequency modulation receiver. Signals from antenna 10 are applied to radio frequency amplifier 12, wherein the level of the signals is increased. The amplified signals are ap plied to converter 14, which may include one or more stages of frequency conversion to reduce the signal frequency. The signals from converter 14 are applied to intermediate frequency amplifier 16, and then to limiter 18. The limited signals are applied to discriminator 20 wherein the modulation signals are derived from the frequency modulation wave. The modulation signals are applied through audio amplifier 22 to a loudspeaker 24. The portion of the receiver which has been thus far described may be of standard construction.

The modulation signals from the discriminator 20 are also applied through a capacitor 26 to a decoder circuit 28 which includes a vibratory reed device 30. This device includes a magnetic tine which has dimensions to provide resonance at a predtermined frequency. The reed is excited by signals applied to a driver winding and produces signals in a pickup winding. When the modulation signals include a tone signal at the resonant frequency of the reed device 30, the balanced detector 32 will generate the proper magnitude signals to actuate the electronic switch 34 which in turn provides a control potential for enabling tone oscillator 36. An alerting tone from oscillator 36 is applied to audio amplifier 22 to amplify and apply the same to the loudspeaker 24. 7

Referring now more specifically to the decoder circuit 28, there is provided an NPN reed amplifier transistor 38 having its input electrode or base 40 coupled to the capacitor 26, an emitter 42 coupled to ground, and an output electrode or collector 44 coupled to the primary or driver winding 46 of vibratory reed device 30. Bias for transistor 38 is provided by a resistor 48 coupled from B+ to base 40. When the modulation signals from discriminator 20 includes a tone signal at the resonant frequency of the reed device 30, the tine thereof vibrates and generates a signal at such frequency across the secondary or pickup winding 50. This signal will hereinafter be referred to as the feed through signal because it is applied from capacitor 26 to the balanced detector 32 through the reed device 30. Opposite phases of the feedthrough signal will appear at the respective ends of the pickup winding 50, each having an amplitude equal to one-half the total feedthrough signal across the winding 50.

A second path to the balanced detector 32 is provided by a conductor 52 which is coupled between capacitor 26 and the junction of the pair of resistors 54 and 56 to provide similar phases of the tone signal at the ends of the pickup winding 50. Since this latter signal bypases the read device 30, it will hereinafter be referred to as the bypass signal. Alternatively the bypass signal could also be applied to a center tap on the pickup Winding 50 to provide the same result.

The bypass signal on conductor 52 is individually combined with the oppositely phased feedthrough signals on the respective ends of the pickup winding 50. Since transistor 38 provides 180 phase shift and assuming reed device 30 provides no phase shift, the bypass and feedthrough signals on the top end of pickup winding 50 will actually subtract to provide a difference (meaning the sum of two signals 180 out of phase with each other) signal while the signals on the bottom end of winding 50 will add to provide a sum signal.

The balanced detector 32 in decoder circuit 28 includes a first channel having an NPN detector transistor 58 and a second channel having an NPN detector transistor 60. The emitters thereof are connected to ground and the bases 62 and 64 are respectively coupled to the top and bottom ends of pickup winding 50. The resistor 48 which provides bias for the transistor 38 also provides bias for transistors 58 and 60 via the direct current coupling path consisting of resistors 54 and 56 to the bases 62 and 64. Such bias may either be sufficient to saturate both transistors, or bias them into class A operation, or into class C operation, but in any case the conduction of the transistors will be determined by the amplitudes of the sum and difference signals as will be more fully explained hereinafter. The collectors 66 and 68 of transistors 58 and 60 are respectively coupled through load resistors 70 and 72 to B+. The conduction of transistor 58 will be related to the amplitude of the difference signal on its base so that the voltage on collector 66 will be an amplified representation thereof. An integrating network for transistor 58 includes the load resistor 70 and a capacitor 74 connected between collector 66 and ground. The time constant of such network is selected to be long compared to the inverse of the tone signal frequency in order to detect the average component of the amplified representation on collector 66 and thereby provide a DC difference voltage related to the magnitude of the difference signal. Similarly, the integrating network for transistor 60 formed by load resistor 72 and capacitor 76 has a similarly selected time constant to provide a DC sum voltage on collector 68 related to the magnitude of the sum signal on base 64.

The DC sum voltage on collector 66 is applied through a resistor 78 to the base 80 of an NPN transistor 82 included within the electronic switch 34, and the DC difference voltage on collector 68 is applied through resistor 84 to the emitter 86 of the transistor 82. Resistors 78 and 84 and a capacitor 88 provide additional filtering for the DC sum and difference voltages. When the frequency of the tone signal is at the resonant frequency of the reed device 30, the DC difference voltage has a maximum positive value and the DC sum voltage has a minimum positive value to render transistor 82 conductive. This causes current to flow from B+ through the emitter-base junction of a PNP transistor 90 through the collectoremitter junction of transistor 82 so that transistor 90 becomes saturated, whereby a 13-}- control potential appears on the collector 92 of transistor 90 and also on the output conductor 94 of the decoder circuit 28. The control potential is further filtered by a capacitor 95. Such potential is fed back through a resistor 94 to the base 80 of transistor 82 to regeneratively maintain transistors 82 and 90 conductive as long as the DC sum and difference voltages are applied to the transistor 82 which, in turn, is as long as the tone signal is present. Tone oscillator 36 is constructed to be enabled by the B+ control potential to generate an alerting tone which is amplified by the audio amplifier 22 and applied to loudspeaker 24.

The operation just explained may be more clearly understood by referring to FIG. 2, where the bypass signal 98 is 180 out-of-phase with the feedthrough signal 100 at the top end of pickup winding 50 so that when the two signals are added together, difference signal 102 having an amplitude of zero is provided. For illustration purposes it will be assumed that the transistors 58 and 60 are biased into class C operation, that is when no signal is applied to the transistors, the respective collectors are at the B+ voltage and as the signals applied thereto increase in the positive direction, the collector voltage decreases from B+. Therefore, when difference signal 102 is applied to transistor 58, the DC difference voltage 103 on the collector 66 will be equal to B+.

The bypass signal 98 is also applied to the bottom end of pickup winding 50, but since feedthrough signal 104 is 180 out-of-phase with the feedthrough signal 100, signals 98 and 104 will add to provide a sum signal 106 with the negative portions thereof maintaining the transistor 60 non-conductive and with the positive portions thereof being of a sufficient value to saturate the transistor and to provide a half wave rectified signal 108 on collector 68. The integrating network consisting of resistor 72 and capacitor 76 detects the average component of signal 108 to provide a DC sum voltage 110 having an amplitude approximately 2B+/ 3 (the average of a halfwave rectified signal measured from its base is 1/1r).

The difference between voltages 103 and 110, here B+/3, is applied between the base and emitter of transistor 82 and acts to bias on transistor 82 and therefore actuate the switch 34 to provide the necessary B+ control potential on conductor 96 to enable oscillator 36.

" Of course, the sum signal 106 may be of sufficient amplitude to drive the peaks of the half wave rectified signal 108 beyond saturation to reduce the DC sum voltage 110, or alternatively the amplitude of the sum signal may be less than that shown, in which case, the DC voltage applied between the base and emitter will be higher.

The gain of the reed amplifier transistor 38 may compensate for the insertion loss of the reed device 30 plus the loss" due to the feedthrough signals having only onehalf their total amplitude, so that the feedthrough signals 100 and 104 may be equal in amplitude to the bypass signal 98, as is the case shown in FIG. 2. It should be noted, however, that other relationships between the signals will work as well.

FIG. 3 illustrates the frequency response of a resonant circuit, here the reed device 30, having a resonant frequency f In the explanation just concluded it was assumed that the frequency of the tone signal applied to the reed amplifier transistor 38 was precisely equal to the resonant frequency f However, it is usually desirable that the decoder circuit also responds to a limited range of frequencies centered about f in order to compensate for inaccuracies in transmission of the tone signals and in construction of the reed device 30. Assume that the decoder circuit 28 is to enable tone oscillator 36 (FIG. 1) for any tone signal having a frequency within the range 112. At the limits of range 112, the amplitude of the feedthrough signal is one-half the amplitude of such signal when tone signal frequency was i How this is achieved is shown in FIG. 4 where the tone signal and thus the bypass signal 114 has a frequency at the lower limit of the range 112 (the upper limit would yield the same results), but due to the operation of the limiter 18, its amplitude is the same as the frequency of bypass signal 98 of FIG. 2. The feedthrough signal 116 which has an amplitude one-half the amplitude of the corresponding signal of FIG. 2, as previously assumed, adds to bypass signal 114. The difference signal 118 thereby developed maintains the transistor 58 non-conductive for the first half cycle and renders the transistor slightly conductive during the second half cycle to provide a half wave rectified signal 120 having an average DC difference voltage 122 slightly less than B+.

The sum signal 124 resulting from the addition of the bypass signal 114 and the oppositely phased feedthrough signal 126 is of sufiicient value during its second half cycle to render transistor 60 substantially conductive to through signals 116 and 126 have amplitudes half their provide an average DC sum voltage 128. Since the feedcounterparts on resonance, it can be shown that the difference between the DC voltages 122 and 128 is one-half the corresponding difference in FIG. 2, or B+/6. By properly adjusting the quiescent bias on transistor 82 and the gains of the various transistors in the decoder circuit 28 so that B+/6 or greater actuates the switch 34, then an alerting tone will be produced whenever the frequency of the tone signal is within the range 112 of FIG. 3. When the difference is less than B+/6, as is the case when the tone signal frequency is further removed from f the switch 34 is not actuated and the control potential on the output conductor 96 remains at ground to maintain tone oscillator 36 disabled so that no alerting tone is produced outside of range 112.

In order to facilitate the explanation of FIG. 4, it was assumed that bypass signal 114 and feedthrough signal 126 were precisely in phase, and signal 114 and feedthrough signal 126 were precisely 180 out-of-phase whereas in practice, due to the phase shifting characteristic of a resonant circuit, as the frequency of the input tone signal deviates from the resonant frequency thereof, the relative phases between the feedthrough and bypass signal changes. Thus in order to be perfectly accurate, it would be necessary to vectorially add and subtract the signals rather than add and subtract algebraically. However, the actual voltages provided do not vary significantly from the voltages illustrated which are believed to adequately define the operation.

Completing the analysis, assume that the frequency of the tone signal is sufficiently removed from the resonant frequency of the reed device 30 such that the amplitudes of the feedthrough signals are substantially zero and therefore only the bypass signals 130 and 132 shown in FIG. 5 are applied to the respective bases of transistors 58 and 60. The half wave rectified signals 134 and 136 respectively developed on the collectors 66 and 68 have the same average DC voltages 138 and 140 so that the difference is zero and therefore the switch 34 will not be actuated.

' Although the average DC voltages on the collectors of the transistors 58 and 60 are shown to be coupled respectively to the base and emitter of the transistor 82, such voltages may be applied to, for example, a differential combiner which provides a voltage proportional to their difference which in turn may be used to actuate a switch. It is also to be noted that a variety of polarity signals and conductivity transistors other than that described may be used. For example, detector transistors 58 and 60 may be quiescently biased into saturation in which case, and referring to FIG. 2, the DC difierence voltage 103 would be zero instead of B+ and the DC sum voltage 108 would be B+/3. In such case the transistors in the electronic switch 34 should have opposite conductivities that is transistor 82 would be a PNP type and transistor 92 would be an NPN type, and the polarity of the DC supply voltages would have to be changed.

A most important feature in this invention is the low sensitivity of the decoder circuit 28 and therefore the tone oscillator 36 to produce false alerting tones when the receiver is dropped or otherwise subjected to shock. Referring to FIG. 6, when the reed device 30 is physically shocked, the time therein vibrates to provide oppositely phased signals 142 and 144, the amplitude of which is proportional to the force of the vibration. Since there is no bypass signal to add thereto, they alone will control the conduction of the transistors 58 and '60 respectively such that the half wave rectified signals 146 and 148 are developed on the respective collectors 66 and 68. Even though the transistors 58 and 60 conduct on alternate half cycles, the average DC voltages 150 and 152 detected by the integrating networks are equal so that the switch 34 is not actuated. Accordingly, output conductor 96 of decoder circuit 28 remains at ground and tone oscillator 36 remains disabled so that no alerting tone is produced by the speaker 24. An advantageous characteristic of this circuit is the fact that either a slight physical shock or an extreme physical shock will not enable the tone oscillator 36 because in all cases the average DC voltage will be equal.

A second embodiment of the invention is shown in FIG. 7 where like parts are labeled with the same reference numerals. Here conductor 52 is coupled to the base of an amplifier transistor 150 which amplifies the tone signal to provide a bypass signal on the emitters of transistors 58 and 60. B+ for the collector of transistors 150 is provided through a pair of resistors 152 and 154. A capacitor 156 may be placed at the junction between resistors 54 and 56 to prevent any AC signal from appearing thereat. Now the bypass signal is shifted by transistor 150 and is therefore in phase with the feedthrough signal applied to the base 62 of transistor 58 so that the amplitude of the average DC voltage on the collector 66 of transistor 58 is again proportional to the difference between the signals, and similarly the amplitude of the DC voltage on the collector 68 of transistor 60 is proportional to the sum. Thus FIGS. 2-6 are similarly applicable to the decoder circuit of FIG. 7. In either the FIG. 1 or FIG. 7 embodiment, the basic operation involves applying the bypass signal and one phase of the feedthrough signal to one detector transistor to derive the DC difference voltage, and applying the bypass signal and an opposite phase of the feedthrough signal to the other detector transistor to derive the DC sum voltage.

It may be appreciated that the invention, although described as responsive to a single signal tone, may be used, for example, where the receiver is to respond to a plurality of tones whether sequentially or simultaneously transmitted, by utilizing the control potential on the output conductor 96 of the decoder circuit 28 for controlling further circuits or by using the difference between the average DC voltages on the collectors of transistors 58 and 60 for a similar purpose.

What has been described, therefore, is a tone decoder circuit to provide an alerting tone in response to a tone signal within a limited range of frequencies and including as an integral part thereof, means to minimize the susceptibility of the decoder circuit to false alerting by physical shock of the reed device.

I claim:

1. A tone decoder circuit to respond to tone signals within a limited range of frequencies centered about a selected frequency to provide a control signal, which decoder circuit includes in combination; a vibratory reed device having a resonant frequency approximately equal to the selected frequency and having driver and pickup windings, input circuit means for applying the tone signal to said driver winding to actuate the reed device and cause a reed output signal to appear across said pickup winding, circuit means coupled to said input circuit and bypassing said reed device for providing a bypass signal at the frequency of the tone signal, adder means including a first detector coupled to said pickup winding and to said circuit means to provide a first direct current voltage of a value related to the magnitude of the sum of said reed output signal and said bypass signal, subtractor means including a second detector coupled to said pickup winding and to said circuit means to provide a second direct current voltage of a value related to the magnitude of the sum of said reed output signal and said bypass signal, and means coupled to said added means and said subtractor means and responsive to the differential combination of said first and second direct current voltages to provide a control signal having a first level when the frequency of the tone signal is within the limited range of frequencies and a second level when the frequency of the tone signal is outside such limited range.

2. A tone decoder circuit to respond to tone signals within a limited range of frequencies centered about a selected frequency to provide a control signal, which decoder circuit includes in combination; a vibratory reed device having a resonant frequency approximately equal to the selected frequency and having driver and pickup windings, amplifier means having an input circuit for receiving the tone signal and an output circuit coupled to said driver winding, said pickup winding having first and second ends which respectively receive first and second signals of opposite phases at the resonant frequency in response to activation of the reed device by the tone signal, circuit means coupled to said input circuit for providing a third signal at the frequency of the tone signal, tone frequency detector means having a first channel coupled to said first end and to said circuit means to provide a first direct current voltage of a value related to the magnitude of the sum of said first and third signals, said detector means having a second channel coupled to said second end and to said circuit means to provide a second direct current voltage of a value related to the magnitude of the sum of said second and third signals, said first and second channels each having an output conductor on which said direct current voltages appear, and switch means coupled to said output conductors and responsive to the differential combination of said first and second direct current voltages to provide a control signal having a first level when the frequency of the tone signal is within the limited range of frequencies and a second level when the frequency of the tone signal is without such limited range.

3. The tone decoder circuit as set forth in claim 2, wherein said circuit means includes first resistor means coupled to the junction of said first channel and the first end of said pickup winding, and second resistor means coupled to the junction of second channel and the second end of said pickup winding, with said resistor means being coupled together and electrically coupled to said input circuit for applying said third signal to each of said channels.

4. The tone decoder as set forth in claim 2, wherein each of said channels includes a detector device each having first and second input electrodes, said first end of said secondary winding being coupled to the first input electrode of the detector device in said first channel, said second end being coupled to the first input electrode of the detector device in said second channel, said input circuit being electrically coupled through said circuit means to said second input electrode of the detector devices in each of said channels.

5. The tone decoder as set forth in claim 4 wherein said circuit means includes an amplifier device.

6. A tone decoder circuit to respond to tone signals within a limited range of frequencies centered about a selected frequency to provide a control signal, which decoder circuit includes in combination; a vibratory reed device having a resonant frequency approximately equal to the selected frequency and having primary and secondary windings, tone signal supply means, amplifier means having an input electrode coupled to said supply means and an output electrode coupled to said primary winding, said secondary winding having first and second ends whereat oppositely phased signals at the resonant frequency respectively appear when a tone signal is applied to said input electrode and also when said reed device is shock excited, reed device bypassing means electrically coupling said supply means to said secondary winding for respectively providing similar phased signals on the ends of said secondary winding only when a tone signal is applied to said input electrode to individually combine with the oppositely phased signals to provide a sum signal on said first end of said secondary winding and a difference signal on said second end thereof, tone frequency detector means having a first channel coupled to said first end to provide a direct current voltage of a value related to the magnitude of said sum signal, said detector means having a second channel coupled to said secend end to provide a direct current voltage of a value related to the magnitude of said difference signal, said first and second channels each having an output conductor on which said direct current voltages appear, and switch means coupled to said output conductors and responsive to the differential combination of said direct current voltages to provide a control signal having a given level when the frequency of the tone signal is within the limited range of frequencies and having another level when said reed device is shock excited and when the frequency of the tone signal is without the limited range of frequencies.

7. The tone decoder as set forth in claim 6 wherein said reed device bypassing means includes first resistor means coupled to the junction of said first channel and the first end of said secondary winding, and second resistor means coupled to the junction of said second channel and the second end, of said secondary winding, both of said resistor means being coupled together and electrically coupled to said tone signal supply means.

8. The tone decoder as set forth in claim 6 wherein said amplifier means includes a transistor and bias means coupled to said input electrode to establish a bias for said transistor, said first channel including a first detector transistor having an input electrode coupled to said first end of said secondary winding, said second channel including a second detector transistor having an input electrode coupled to said second end thereof, said reed device bypassing means including first resistor means direct current coupled to the input electrode of said first detector transistor, and second resistor means direct current coupled to the input electrode of said second detector transistor, each of said resistor means being direct current coupled together and direct current coupled to said bias means to establish a bias for said detector transistor.

9. The tone decoder as set forth in claim 6 wherein each of said channels includes a transistor having an input and an output electrode, each of said channels further including an integrating circuit coupled to the respective output electrodes and having a time constant long with respect to the inverse of the frequency of the tone signal to respectively convert said sum and difference signals into direct current voltages.

10. The tone decoder set forth in claim 6 further including oscillator means coupled to said switch means and responsive to said control signal being equal to said given level to produce an alerting signal.

References Cited UNITED STATES PATENTS 3,341,815 9/1967 Axe 340-171 JOHN W. CALDWELL, Primary Examiner H. I. PITTS, Assistant Examiner U.S. Cl. X.R. 

